arXiv subjects

Tom Ichibha

Publications and source records attributed to Tom Ichibha.

At least 19 recordsLinked to original sources

arXiv2026

Application and Performance Assessment of Annealing Methods for Electrostatic-Energy-Based Configuration Search in Mixed Crystals

In first-principles design of solid solutions and disordered materials, exhaustive evaluation of all possible substitutional configurations is often impractical because the number of site occupations increases exponentially. Here, we develop a framework for pre-screening mixed-crystal configurations using annealing methods, where the Ewald electrostatic energy is used as the objective function. Substitutional occupations are represented by binary variables, allowing the Ewald energy to be mapped onto an Ising-type Hamiltonian and the search for low-energy configurations to be formulated as a combinatorial optimization problem. We implement this formulation using simulated annealing (SA) and quantum annealing (QA), and benchmark their performance against exhaustive search. For the small-scale system CaYAlO$_4$, SA achieved a speed-up of about 30 times, while QA achieved a speed-up of more than 100 times; both methods identified all lowest-energy configurations. For the medium-scale system $\beta$-KSbF$_4$ and the large-scale Ba-doped SiAlON system, SA achieved speed-ups of about 200-300 times while robustly identifying the lowest-energy structures. In contrast, QA was effective for the small-scale case but showed limited speed-up for medium-scale problems and missed some low-energy configurations due to chain breaks. These results indicate that SA is currently the most robust and general-purpose approach for rapid pre-screening of mixed-crystal configurations based on electrostatic energy. The proposed formulation can be implemented automatically using publicly available libraries and provides a practical route for accelerating candidate-structure generation before first-principles calculations.

cond-mat.mtrl-sci↗

arXiv2025

Empowering Neural Network-based Quantum Monte Carlo with Local Pseudopotentials

Neural Network-based Quantum Monte Carlo (NNQMC), an emerging method for solving many-body quantum systems with high accuracy, has been mainly applied to small systems due to demanding computation requirements. In this work, we introduce a framework based on local pseudopotentials to break through such limitation, improving the computational efficiency and scalability of NNQMC. The incorporation of local pseudopotentials reduces the number of electrons treated in neural network and also achieves better relative energy accuracy than all electron NNQMC calculations for complex systems. This counterintuitive outcome is made possible by the distinctive characteristics inherent to NNQMC. Notably, by avoiding costly integration terms, this approach is also substantially more efficient than its widely used semilocal counterparts. Our approach enables the reliable treatment of large and challenging systems, such as the $\text{Fe}_4 \text{S}_4 (\text{SCH}_3)_4$ iron-sulfur cluster. Overall, our findings demonstrate that the synergy between NNQMC and local pseudopotentials substantially expands the scope of accurate ab initio calculations.

physics.chem-ph↗

arXiv2025

Electronic transport properties of titanium nitride grown by molecular beam epitaxy

This study investigates the molecular beam epitaxial (MBE) growth of titanium nitride (TiN) thin films, achieving a high residual resistivity ratio (RRR) of 15.8. We observed a strong correlation between growth temperature and crystalline quality, as reflected in both RRR values and lattice parameter variations. Characterization of superconductivity yielded a Ginzburg-Landau coherence length of 60.4 $\pm$ 0.6 nm, significantly higher than typical sputtered films, suggesting improved superconducting coherence. First-principles calculations, in conjunction with experimental data, provided detailed insights into the electronic structure and transport properties of the TiN films. Temperature-dependent Hall coefficient measurements further revealed the influence of anisotropic scattering mechanisms. These findings establish a promising route for the development of nitride-based superconducting materials for advanced quantum computing technologies.

cond-mat.supr-con↗

arXiv2025

Ab initio screening for BCS-type superconductivity in ThCr$_2$Si$_2$-type compounds

In this study, we applied ab initio $T_\mathrm{c}$ calculations to compounds with the ThCr$_2$Si$_2$-type structure to search for BCS superconductor candidates. From the 1883 compounds registered in the Inorganic Crystal Structure Database, we excluded those whose chemical compositions would inhibit the emergence of BCS-type superconductivity by giving rise to magnetism or heavy-fermionic behavior. We then focused on 66 compounds confirmed to be dynamically stable through phonon calculations. Among these, for the 24 systems with experimentally reported $T_\mathrm{c}$ values, we verified that the ab initio $T_\mathrm{c}$ calculations exhibit excellent predictive reliability. For the remaining 42 compounds lacking experimental $T_\mathrm{c}$ values, our predictions identified several new BCS-type superconductor candidates, including SrPb$_2$Al$_2$ $\left(T_c^\mathrm{calc}=2.2\,\mathrm{K}\right)$.

cond-mat.supr-con↗

arXiv2023

Locality Error Free Effective Core Potentials for 3d Transition Metal Elements Developed for the Diffusion Monte Carlo Method

Pseudopotential locality errors have hampered the applications of the diffusion Monte Carlo (DMC) method in materials containing transition metals, in particular oxides. We have developed locality error free effective core potentials, pseudo-Hamiltonians, for transition metals ranging from Cr to Zn. We have modified a procedure published by some of us in [M.C. Bennett et al, JCTC 18 (2022)]. We carefully optimized our pseudo-Hamiltonians and achieved transferability errors comparable to the best semilocal pseudopotentials used with DMC but without incurring in locality errors. Our pseudo-Hamiltonian set (named OPH23) bears the potential to significantly improve the accuracy of many-body-first-principles calculations in fundamental science research of complex materials involving transition metals.

cond-mat.mtrl-sci↗

arXiv2023

Control of Impurity Phase Segregation in a PdCrO$_2$/CuCrO$_2$ Heterostructure

PdCrO$_2$ films are synthesized on CuCrO$_2$ buffer layers on Al$_2$O$_3$ substrates. This synthesis is accompanied by impurity phase segregation, which hampers the synthesis of high quality PdCrO$_2$ films. The potential causes of impurity phase segregation were studied by using a combination of experiments and ab initio calculations. X-ray diffraction and scanning transmission electron microscopy experiments revealed impurity phases of Cu$_x$Pd$_{1-x}$ alloy and chromium oxides, Cr$_2$O$_3$ and Cr$_3$O$_4$, in PdCrO$_2$. Calculations determined that oxygen deficiency can cause the impurity phase segregation. Therefore, preventing oxygen release from delafossites could suppress the impurity phase segregation. The amounts of Cr$_2$O$_3$ and Cr$_3$O$_4$ depend differently on temperature and oxygen partial pressure. A reasonable theory-based explanation for this experimental observation is provided.

cond-mat.mtrl-sci↗

arXiv2022

Anionic Ordering in Pb$_2$Ti$_4$O$_9$F$_2$ Revisited by Nuclear Magnetic Resonance and Density Functional Theory

A combination of $^{19}$F magic angle spinning (MAS) nuclear magnetic resonance (NMR) and density functional theory (DFT) were used to study the ordering of F atoms in Pb$_2$Ti$_4$O$_9$F$_2$. This analysis revealed that F atoms predominantly occupy two of the six available inequivalent sites in a ratio of 73:27. DFT-based calculations explained the preference of F occupation on these sites and quantitatively reproduced the experimental occupation ratio, independent of the choice of functional. We concluded that the Pb atom's 6$s^2$ lone pair may play a role ($\sim$0.1 eV/f.u.) in determining the majority and minority F occupation sites with partial density of states and crystal orbital Hamiltonian population analyses applied to the DFT wave functions.

cond-mat.mtrl-sci↗

arXiv2021

Candidate structure for the H$_2$-PRE phase of solid hydrogen

Experimental progress finally reached the metallic solid hydrogen phase, which was predicted by Wigner and Huntington over 80 years ago. However, the different structures in the phase diagram are still been debated due to the difficulty of diffraction experiments for high-pressured hydrogen. The determination of crystal structures under extreme condition is both of the basic condensed matter physics, and in planetary science: the behavior of giant gaseous planets (e.g. Jupiter, Saturn...) strongly depends on the properties of inner high-pressured hydrogen. This work describes new possible structures appearing under high pressures of 400$\sim$600 GPa. We applied a structural search using particle swarm optimization with density functional theory (DFT) to propose several candidate structures. For these structures, we performed fixed-node diffusion Monte Carlo simulations combined with DFT zero-point energy corrections to confirm their relative stability. We found $P2_{1}/c$-8 as a promising candidate structure for the H$_2$-PRE phase. $P2_{1}/c$-8 is predicted the most stable at 400 and 500~GPa. $P2_{1}/c$-8 reproduces qualitatively the IR spectrum peaks observed in the H$_2$-PRE phase.

cond-mat.mtrl-sci↗

arXiv2021

Diffusion Monte Carlo Study on Relative Stabilities of Boron Nitride Polymorphs

Although Boron nitride (BN) is a well-known compound widely used for engineering and scientific purposes, the phase stability of its polymorphs, one of its most fundamental properties, is still under debate. The ab initio determination of the ground state of the BN polymorphs, such as hexagonal and zinc-blende, is difficult because of the elusive Van der Waals interaction, which plays a decisive role in some of the polymorphs, making quantitative prediction highly challenging. Hence, despite multiple theoretical studies, there has been no consensus on the ground state yet, primarily due to contradicting reports. In this study, we apply a state-of-the-art ab initio framework - fixed-node diffusion Monte Carlo (FNDMC), to four well known BN polymorphs, namely hexagonal, rhombohedral, wurtzite, and zinc-blende BNs. Our FNDMC calculations show that hBN is thermodynamically the most stable among the four polymorphs at 0 K as well as at 300K. This result agrees with the experimental data of Corrigan~{\it et al.} and Fukunaga. The conclusions are consistent with those obtained using other high-level methods, such as coupled cluster. We demonstrate that the FNDMC is a powerful method to address polymorphs that exhibit bonds of various forms. It also provides valuable information, like reliable reference energies, when reliable experimental data are missing or difficult to access. Our findings should promote the application of FNDMC for other van der Waals materials.

cond-mat.mtrl-sci↗

arXiv2021

Diffusion Monte Carlo evaluation of disiloxane linearization barrier

The disiloxane molecule is a prime example of silicate compounds containing the Si-O-Si bridge. The molecule is of significant interest within the field of quantum chemistry, owing to the difficulty in theoretically predicting its properties. Herein, the linearisation barrier of disiloxane is investigated using a fixed-node diffusion Monte Carlo (FNDMC) approach, which is currently the most reliable {\it ab initio} method in accounting for an electronic correlation. Calculations utilizing the density functional theory (DFT) and the coupled cluster method with single and double substitutions, including noniterative triples (CCSD(T))are carried out alongside FNDMC for comparison. Two families of basis sets are used to investigate the disiloxane linearisation barrier - Dunning's correlation-consistent basis sets cc-pV$x$Z ($x = $ D, T, and Q) and their core-valence correlated counterparts, cc-pCV$x$Z. It is concluded that FNDMC successfully predicts the disiloxane linearisation barrier and does not depend on the completeness of the basis sets as much as DFT or CCSD(T), thus establishing its suitability.

physics.chem-ph↗

arXiv2020

A new ab initio modeling scheme for ion self-diffusion coefficient applied for ε-Cu3Sn phase of Cu-Sn alloy

We present a new modeling scheme for ion self-diffusion coefficient, which broadens the applicable scope of ab initio approach. The essential concepts of the scheme are `domain division' and `coarse graining' of the diffusion network based on the barrier energies predicted by the ab initio calculation. The scheme was applied to evaluate Cu ion self-diffusion coefficient in ε-Cu3Sn phase of Cu-Sn alloy, which is a typical system having long-range periodicity. The model constructed with the scheme successfully reproduces the experimental values in a wide temperature range.

cond-mat.mtrl-sci↗

arXiv2020

Quantum annealing approach to Ionic Diffusion in Solid

We have developed a framework for using quantum annealing computation to evaluate a key quantity in ionic diffusion in solids, the correlation factor. Existing methods can only calculate the correlation factor analytically in the case of physically unrealistic models, making it difficult to relate microstructural information about diffusion path networks obtainable by current ${ab\ initio}$ techniques to macroscopic quantities such as diffusion coefficients. We have mapped the problem into a quantum spin system described by the Ising Hamiltonian. By applying our framework in combination with ab initio technique, it is possible to understand how diffusion coefficients are controlled by temperatures, pressures, atomic substitutions, and other factors.We have calculated the correlation factor in a simple case with a known exact result by a variety of computational methods, including simulated quantum annealing on the spin models, the classical random walk, the matrix description, and quantum annealing on D-Wave with hybrid solver. This comparison shows that all the evaluations give consistent results with each other, but that many of the conventional approaches require infeasible computational costs. Quantum annealing is also currently infeasible because of the cost and scarcity of Q-bits, but we argue that when technological advances alter this situation, quantum annealing will easily outperform all existing methods.

quant-ph↗

arXiv2020

Ab initio evaluation of complexation energies for cyclodextrin drug inclusion complexes

We examined the reliability of exchange-correlation functionals for molecular encapsulations combined by van der Waals forces, comparing their predictions with those of diffusion Monte Carlo method. We established that functionals with D3 dispersion force correction and including sufficient proportion of exact-exchange in long-ranged interaction can comparatively reliably estimate the binding strength. Our finding agrees with a previous ab initio study on argon dimer. However we found that even such functionals may not be able to distinguish the energy differences among different conformations.

physics.chem-ph↗

arXiv2019

Inconsistencies in ab initio evaluations of non-additive contributions of DNA stacking energies

We evaluated the non-additive contributions of the inter-molecular interactions in B-DNA stacking by using diffusion Monte Carlo methods with fixed node approximations (FNDMC). For some base-pair steps, we found that their non-additive contributions evaluated by FNDMC significantly differ from those by any other {\it ab initio} methods, while there are no remarkable findings on their stacking energies themselves. The apparently unexpected results of non-additivity raise issues in both FNDMC and correlated wavefunction methods. For the latter, it can be partly attributed to the imperfect complete basis set (CBS) correction scheme due to the limitation of the computational costs. On the other hand, the striking contrast between the stacking and non-additivity behaviors was found in FNDMC. This might imply that the error cancellations of the fixed node biases in FNDMC work well for the stacking energies, while not for the non-additivity contributions involving charge transfers caused by hydrogen bonds bridging Watson-Crick base pairs.

physics.chem-ph↗

arXiv2019

Ti interstitial flows giving rutile TiO$_2$ reoxidation process enhanced in (001) surface

We revisited ab initio evaluations of the energy barriers along the possible diffusion paths of the defects in rutile TiO$_2$. By using a method carefully considering the cancellation of the self-interaction, Ti interstitials hopping along $c$-axis are identified as the major diffusion directing to [001] surface. The conclusion is contradicting to any of previous theoretical works, and the discrepancy is explained by the overestimation of the radius of defects due to the poor cancellations in the previous works. The updated prediction here can explain the superior photocatalysis activity in [001] surface to [110].

cond-mat.mtrl-sci↗

arXiv2019

Making the most of data: Quantum Monte Carlo Post-Analysis Revisited

In quantum Monte Carlo (QMC) methods, energy estimators are calculated as the statistical average of the Markov chain sampling of energy estimator along with an associated statistical error. This error estimation is not straightforward and there are several choices of the error estimation methods. We evaluate the performance of three methods, Straatsma, an autoregressive model, and a blocking analysis based on von Neumann's ratio test for randomness, for the energy time-series given by Diffusion Monte Carlo, Full Configuration Interaction Quantum Monte Carlo and Coupled Cluster Monte Carlo methods. From these analyses we describe a hybrid analysis method which provides reliable error estimates for series of all lengths. Equally important is the estimation of the appropriate start point of the equilibrated phase, and two heuristic schemes are tested, establishing that MSER (mean squared error rule) gives reasonable and constant estimations independent of the length of time-series.

physics.comp-ph↗

arXiv2019

Importance of vdW and long-range exchange interactions to DFT-predicted docking energies between plumbagin and cyclodextrins

We calculated the docking energies between plumbagin and cyclodextrins, using density functional theory (DFT) with several functionals and some semi-empirical methods. Our DFT results revealed that GD3 dispersion force correction significantly improves the reliability of prediction. Also sufficient amount of long-range exchange is important to make it reliable further, agreeing with the previous work on argon dimer. In the semi-empirical methods, PM6 and PM7 qualitatively reproduce the stabilization by docking , yet under- and over-estimating the docking energies by ~10 kcal/mol, respectively.

cond-mat.mtrl-sci↗

arXiv2018

$\boldsymbol{Ab\ initio}$ search of polymer crystals with high thermal conductivity

Lattice thermal conductivities (LTC) for a subset of polymer crystals from the Polymer Genome Library were investigated to explore high LTC polymer systems. We employed a first-principles approach to evaluating phonon lifetimes within the third-order perturbation theory combined with density functional theory, and then solved the linearized Boltzmann transport equation with single-mode relaxation time approximated by the computed lifetime. Typical high LTC polymer systems, polyethylene (PE) crystal and fiber, were benchmarked, which is reasonably consistent with previous references, validating our approach. We then applied it to not only typical polymer crystals, but also some selected ones having structural similarities to PE. Among the latter crystals, we discovered that beta phase of Poly(vinylidenesurely fluoride) (PVF-$β$) crystal has higher LTC than PE at low temperature. Our detailed mode analysis revealed that the phonon lifetime of PVF-$β$ is more locally distributed around lower frequency modes and four-times larger than that of PE. It was also found from a simple data analysis that the LTC relatively correlates with curvature of energy-volume plot. The curvature would be used as a descriptor for further exploration of high LTC polymer crystals by means of a data-driven approach beyond human-based one.

cond-mat.mtrl-sci↗